TRACHEATA. 



329 



am. 



each side of it gradually approach each other. They do so at first 

 behind, and then in the middle ; from the latter point the approxi- 

 mation gradually extends backwards and forwards (fig. 176 B and C). 

 In the middle and hinder parts of the ventral plate the groove 

 becomes, by the coalescence of the folds, converted into a canal 

 (fig. 178 A, gg), the central cavity of which soon disappears, while 

 at the same time the cells of the wall undergo division, become more 

 rounded, and forma definite layer .(me) the mesoblast beneath the 

 columnar cells of the surface. Anteriorly the process is slightly 

 different, though it leads to the similar formation of mesoblast (fig. 

 177 B). The flat floor of the groove becomes in front bodily converted 

 into the mesoblast, but the groove itself is never converted into a 

 canal. The two folds simply meet above, and form a continuous 

 superficial layer. 



During the later stages of the process last described remarkable 

 structures, eminently characteristic of the Insecta, have made their 

 first appearance. These structures are certain embryonic membranes or 

 coverings, which present in their mode of formation and arrangement 

 a startling similarity to the true and false amnion of the Vertebrata. 

 They appear as a double fold of the blastoderm round the edge of 

 the germinal area, which spreads over the ventral plate, from behind 

 forwards, in a general way in 

 the same manner as the am- 

 nion in, for instance, the 

 chick. The folds at their 

 origin are shewn in surface 

 view in fig. 176 D, am, and 

 in section in fig. 177 B, am. 

 The folds eventually meet, 

 coalesce (fig. 178, am) and 

 give rise to two membranes 

 covering the ventral plate, 

 viz. an inner one, which is 

 continuous with the edge of 

 the ventral plate ; and an 

 outer, continuous with the 

 remainder of the blastoderm. 

 The vertebrate nomenclature 

 may be conveniently em- 

 ployed for these membranes. 

 The inner limb of the fold 

 will therefore be spoken of 

 as the amnion, and the outer 

 one, including the dorsal 

 part of the blastoderm, as 

 the serous envelope 1 . A 



1 The reverse nomenclature to this is rather inconveniently employed by Metsch- 

 uikoff. 



FIG. 178. SECTIONS THROUGH TWO EMBRYOS 

 OF HYDROPHILUS PICEUS. (After Kowalevsky.) 



A. Section through the posterior part of the 

 embryo fig. 176 D, shewing the completely closed 

 amnion and the germinal groove. 



B. Section through an older embryo in which 

 the mesoblast has grown out into a continuous 

 plate beneath the epiblast. 



gr/. germinal groove ; am. amnion; yk. yolk; 

 ep. epiblast. 



